The Book of Zuhd and Softening of Hearts
وَحَدَّثَنَاهُ يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا وَكِيعٌ، عَنْ إِسْمَاعِيلَ بْنِ أَبِي خَالِدٍ، بِهَذَا الإِسْنَادِ وَقَالَ حَتَّى إِنْ كَانَ أَحَدُنَا لَيَضَعُ كَمَا تَضَعُ الْعَنْزُ مَا يَخْلِطُهُ بِشَىْءٍ .This hadith has been narrated on the authority of Isma'il b. Khalid with the same chain of transmitters and the words are:" One amongst us would relieve himself as the goats do without anything mixing with its excrement
From Sahih Muslim, whose reports the scholars accept as authentic.